Hmm, I'm wondering if this idea came up ever before, but: how about having monsters with native depths of 100 or higher (in addition to the existing monsters, of course)? They would serve to make deep vaults more interesting, especially when comparing them to slightly more shallow ones. E.g. a "monster up to 40 levels out of depth" tile on a DL80 vault could spawn a monster from a depth of up to 120; currently, those tiles will pick monsters from the exact same lists once the player reaches DL60, since no monsters with depths >= 100 exist. And it would also make the postgame more interesting when the player decides to venture down all the way to DL127.

I did that for PWMAngband, as well as redesigning the Ds to have tougher ones below dlvl80.